cmd/5l etc: restore comments lost during C -> Go conversion

It appears that c2go dropped comments inside struct { ... } and enum { ... }.
Restore them.

Identified missing comments by checking for comments present
in the C code but not the Go code, made a list, and then reapplied
with some mechanical help.

Missing comment finder: http://play.golang.org/p/g6qNUAo1Y0

Change-Id: I323ab45c7ef9d51e28eab3b699eb14bee1eef66b
Reviewed-on: https://go-review.googlesource.com/6899
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/internal/obj/objfile.go

@@ -2,6 +2,102 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Writing of Go object files.
+//
+// Originally, Go object files were Plan 9 object files, but no longer.
+// Now they are more like standard object files, in that each symbol is defined
+// by an associated memory image (bytes) and a list of relocations to apply
+// during linking. We do not (yet?) use a standard file format, however.
+// For now, the format is chosen to be as simple as possible to read and write.
+// It may change for reasons of efficiency, or we may even switch to a
+// standard file format if there are compelling benefits to doing so.
+// See golang.org/s/go13linker for more background.
+//
+// The file format is:
+//
+//	- magic header: "\x00\x00go13ld"
+//	- byte 1 - version number
+//	- sequence of strings giving dependencies (imported packages)
+//	- empty string (marks end of sequence)
+//	- sequence of defined symbols
+//	- byte 0xff (marks end of sequence)
+//	- magic footer: "\xff\xffgo13ld"
+//
+// All integers are stored in a zigzag varint format.
+// See golang.org/s/go12symtab for a definition.
+//
+// Data blocks and strings are both stored as an integer
+// followed by that many bytes.
+//
+// A symbol reference is a string name followed by a version.
+// An empty name corresponds to a nil LSym* pointer.
+//
+// Each symbol is laid out as the following fields (taken from LSym*):
+//
+//	- byte 0xfe (sanity check for synchronization)
+//	- type [int]
+//	- name [string]
+//	- version [int]
+//	- flags [int]
+//		1 dupok
+//	- size [int]
+//	- gotype [symbol reference]
+//	- p [data block]
+//	- nr [int]
+//	- r [nr relocations, sorted by off]
+//
+// If type == STEXT, there are a few more fields:
+//
+//	- args [int]
+//	- locals [int]
+//	- nosplit [int]
+//	- flags [int]
+//		1 leaf
+//		2 C function
+//	- nlocal [int]
+//	- local [nlocal automatics]
+//	- pcln [pcln table]
+//
+// Each relocation has the encoding:
+//
+//	- off [int]
+//	- siz [int]
+//	- type [int]
+//	- add [int]
+//	- xadd [int]
+//	- sym [symbol reference]
+//	- xsym [symbol reference]
+//
+// Each local has the encoding:
+//
+//	- asym [symbol reference]
+//	- offset [int]
+//	- type [int]
+//	- gotype [symbol reference]
+//
+// The pcln table has the encoding:
+//
+//	- pcsp [data block]
+//	- pcfile [data block]
+//	- pcline [data block]
+//	- npcdata [int]
+//	- pcdata [npcdata data blocks]
+//	- nfuncdata [int]
+//	- funcdata [nfuncdata symbol references]
+//	- funcdatasym [nfuncdata ints]
+//	- nfile [int]
+//	- file [nfile symbol references]
+//
+// The file layout and meaning of type integers are architecture-independent.
+//
+// TODO(rsc): The file format is good for a first pass but needs work.
+//	- There are SymID in the object file that should really just be strings.
+//	- The actual symbol memory images are interlaced with the symbol
+//	  metadata. They should be separated, to reduce the I/O required to
+//	  load just the metadata.
+//	- The symbol references should be shortened, either with a symbol
+//	  table or by using a simple backward index to an earlier mentioned symbol.
+
 package obj
 
 import (